Under certain operating conditions, aircraft are vulnerable to the accumulation of contaminants on external component surfaces or skins. Examples of such contaminants include ice, water, and mixtures thereof. If left unchecked, the accumulation of ice can eventually so laden the aircraft with additional weight and so alter the airfoil configuration as to cause undesirable flying conditions. The ability to detect the accumulation of ice on such surfaces, and the ability to measure the accumulated thickness thereof so as to identify dangerous flight conditions, has therefore become highly desirable.
A number of different kinds of contaminant detectors have been utilized for such objectives. Among them are capacitive ice detectors, examples of which can be found in U.S. Pat. Nos. 4,766,369 to Weinstein, 5,191,791 to Gerardi et al. and 5,398,547 to Gerardi et al., both of which are hereby incorporated herein by reference.
The Weinstein and Gerardi patents are capacitive type ice detectors. That is, they detect the presence of ice and measures the ice's thickness by measuring changes in capacitance across a pair of spaced electrodes (located flush to the airfoil surface) due to the presence of ice on the airfoil surface between the electrodes.
FIG. 1 is a schematic diagram of an ice detector 10 according to the prior art, including the Weinstein and Gerardi patents. A plurality of capacitance measuring circuits 12, 12' measure the capacitance across a pair of leads 14, 16, 14', 16', respectively, which are connected to a pair of electrodes (not shown). The electrodes and ice can be modeled as RC circuits 18, 18'. Capacitor C.sub.E1, C.sub.E2 represent the polarization capacitance across the electrodes. RC circuits 20, 20' are circuit models of the ice between the electrodes, and are comprised of a Resistor R.sub.I1, R.sub.T2 in parallel with a capacitor C.sub.I1, C.sub.I2. A controller 22 is connected to leads 24 and 26 and interprets the outputs of capacitance measuring circuits 12, 12'. Controller 22 may perform such functions as measure the ratio of capacitance detected by the circuits 12, 12' (as disclosed by Weinstein) or use a computer program to "resolve" ice thickness in some other way (as in Gerardi). One of the techniques suggested for this is to use neural networks and store large data files with capacitance signal profiles of the many different types of contaminants and many different types of ice. Capacitance is then measured and the contaminant classified using the stored data.
Pure ice is relatively nonconductive. R.sub.I is therefore large and the capacitance measurement circuits are effective in reading C.sub.I.
A drawback to the prior art capacitive type detectors is that contaminants other than ice, such as water, are highly conductive. R.sub.I therefore becomes very small and the capacitance measurement circuits are not effective in reading C.sub.I. Also, water causes changes in the overall capacitance across the electrodes similar to changes caused by ice. Since water and glycol by themselves do not create hazardous flying conditions, it is imperative to be able to distinguish between ice and other contaminants. To this end, it is also necessary to be able to identify the presence of ice on top of a layer of water. Because of the aforementioned capacitance measurement problems, Weinstein and Gerardi distinguish between water and ice by either utilizing a temperature probe in conjunction with their capacitive ice detectors, or by changing the stimulation frequency of the capacitance measurement circuit.
Efforts to improve ice detection systems have led to continuing developments to improve their cost, manufacturability, reliability, usefulness, and efficiency.